The invention relates to compositions, methods, and apparatuses for improving drainage retention, wet strength, and dry strength of paper in a papermaking process. A typical papermaking process includes the steps of: 1) pulping wood or some other source of papermaking fibers; 2) producing a paper mat from the pulp, the paper mat being an aqueous slurry of cellulosic fiber which may also contain additives such as inorganic mineral fillers or pigments; 3) depositing this slurry on a moving papermaking wire or fabric; 4) forming a sheet from the solid components of the slurry by draining the water; 5) pressing and drying the sheet to further remove water, and 6) potentially rewetting the dry sheet by passing it through a size press and further drying it to form a paper product.
When conducting a papermaking process, a number of concerns need to be taken into account to assure the quality of the resulting paper product. For example when draining water from the slurry, as many fibers and chemical additives should be retained and not flow out with the water. Similarly the resulting sheet should have adequate wet strength and dry strength.
As described for example in in U.S. Pat. Nos. 7,473,334, 6,605,674, 6,071,379, 5,254,221, 6,592,718, 5,167,776 and 5,274,055 a number of retention aids such as polymers flocculants, and silica based microparticles, may be added to the slurry to facilitate drainage retention. The retention aids function to retain solid matter within the slurry as water is drained out of the slurry. In addition to retaining fibers, the retention aid should also retain additives such as optical brighteners, fillers, and strength agents. The selection of such retention aids is complicated by the fact that they must both allow for the free drainage of water from the slurry and also must not interfere with or otherwise degrade the effectiveness of other additives present in the resulting paper product.
As described for example in U.S. Pat. Nos. 8,465,623, 7,125,469, 7,615,135 and 7,641,776 a number of materials function as effective dry strength agents. These agents can be added to the slurry to increase the strength properties of the resulting sheet. As with retention aids however they must both allow for the free drainage of water from the slurry and also must not interfere with or otherwise degrade the effectiveness of other additives present in the resulting paper product.
As described for example in U.S. Pat. Nos. 8,414,739 and 8,382,947, surface strength agents are materials which increase the resistance of the resulting paper product to abrasive forces. Surface strength agents are often applied as coatings over the formed paper sheet at the size press. Of particular importance is that such agents be compatible with other items present in coatings such as sizing agents and optical brightening agents. In addition desirable surface strength agents must not unduly impair the flexibility of the resulting paper product.
As it is difficult to increase dry strength, surface strength, and/or drainage retention while simultaneously not inhibiting other attributes of the paper or additives therein, there is an ongoing need for improved methods of improving dry strength, surface strength, and/or drainage retention. The art described in this section is not intended to constitute an admission that any patent, publication or other information referred to herein is “prior art” with respect to this invention, unless specifically designated as such. In addition, this section should not be construed to mean that a search has been made or that no other pertinent information as defined in 37 CFR §1.56(a) exists.